Stainless steel and method of coating same



Patented Feb. 12, 1946 STAINLESS STEEL AND METHOD OF COATING SAME IrvineC. Gilligan, Baltimore, Md, assignor, by meme assignments, to TheAmerican Bolling Mill Company, Mlddletown, Ohio, a corporation of OhioNo Drawing.

14 Claims.

, table and corrosion-resistant blackened metallic articles, such asstainless steel articles, having a surface of high quality which retainsits finish dyer a prolonged period of time.

A further object of my invention is the provision of a wide range ofcoated articles such as machine and equipment parts, implements, trim.

fittings. furnishings, articles of apparel, and the pointed outhereinafter.

invention, accordingly, consists in the combination of elements.composition of ingredients and mixture of materials, and in the severalsteps and the relation 01' each of the same to one or more of theothers, and in the products resulting therefrom, as described herein,the scope of the application of which is indicated in the followingclaims.

As conducive to a clearer understanding of certaln features of myinvention. it may be noted at this point that stainless steel is definedas a low-carbon steel comprising 10% to 35% chromium, with or withoutnickel, and with or without supplemental additions of manganese,silicon, cobalt, copper molybdenum tungsten, vanadium, columbium,titanium, and the like, for special purposes. and a balance which issubstantially all iron.

It further may be noted that stainless steel is widely used in the formof products having natural metallic finish. In natural unpolished con-Application December 81, 1942, Serial No. 470.853

, like: 1 "Either objects in part will be obvious and in part been oflimited success. one reason being that the timeofrequiredistoolongtobe',racticai. In this connection. it should be no k that stainless steel isquite stable and does not respond readily to many types of coatingtreatment. oftentimes, the processes necessarily create dangerous orobnoxious fumes or other worldng conditions which are objectionable.Certain known coating moreover. involve the use of chemicals whichdeteriorate in a short time. In such event replacement of the chemicals,usually at considerable expensais quite etsential in-order to iagchieveuniformly treated stainless steel produc I l Some of the heretoforeknown processes are wholly incapable of giving satisfactory coatings onstainless steel products-or, as pointed out hereinbefore, are not easilycontrolled to give an expected quality of coating in a consistentmanner. The coatings obtained often are porous, lack hardness andtoughness, or are insumciently resistant to corrosion. Soft coatings,such as heretofore known hydroxide coatings, are chalky and rub oil orwear away too readily to be of practical use. Tlieyalso undergo change01' appearance in the presence of moisture. Other known coatings are notsufilciently adherent to the underlying metal. 'Ihey crack, chip. orsnail-oil, especially where stressing such as bending or flexing areencountered. Another class of coatings heretofore achieved on stainlesssteel are thick and materially affect the final dimensions of finishedproducts. thus making the products unsuitable for an originally intendeduse requiring accurate dimensions.

An outstanding object of my invention, ac cordingly. is the provision ofa method for producing stainless steel products, or other alloy steelproducts, and the products themselves, which dition the products presenta dull gray or satinlike appearance and, when polished, possess anunusual mirror-like brilliance. There too is wide demand for coatedstainless steel products having surfaces diii'ering in appearance fromthe usual metallic. In meeting this demand, a number of coatingprocesses of the prior art are employed for achieving non-metalliccoatings or films on the metal surfaces. The more popular coatings ofthose heretofore obtained are chemical compounds of stainless steel.which coatings differ from paint or like coatings partly in that theyare more adherent and durable.

The heretofore known chemical processes of coating stainless steel have,for the most part.

coating is stable, durable and corrosion-resistant under a wide varietyof conditions and even is adherent where the metal is bent, whichcoating is hard and tough and uniform both in texture and appearance,and which, being substantially free 01' chalkiness, does not rub oil! orwear away during extended use. 1

Referring now more particularly to the practice of my invention.stainless steel articles, for example, are provided with black oxidecoatings of exceptional quality by immersion in a strong oxidizing bathof sodium dichromate and/or potassium dichromate. The quality andphysical depth of coating obtained are controlled primarily by the timeof immersion and temperature of the immersion bath. As for actualappearance product prior to treatment is important. A dull grayunpolished piece when treated comes out a dull black, while a polishedpiece receives a lustrous black finish as a result of treatment.

As illustrative of the practice of my invention, stainless steelproducts and articles to be coated preferably are given a preliminarycleansing treatment such as pickling to eliminate substantially allscale, oxide film, grease, dirt, or the like often present on theproduct surfaces as a result of earlier treatment or fabrication. Wheredesired, mechanical cleaning methods are resorted to such assandblasting or grinding with or without pickling.

In pickling the stainless steel products, quick dipping in an aqueoussolution of 20% nitric acid and 1% hydrofluoric acid usually issumcient. Thereafter, it is preferable to rinse the products in cleanwater. ilnally obtaining a scalefree and otherwise clean metal surfacewhich usually is of dull gray or satin-like appearance. The productssubsequently are dried and then are ready for coating especially where adull black coated finish is sought.

To prepare the products for receiving a lustrous black finish, I findadvantage in introducing a polishing step before coating. The polishingconveniently is accomplished by bufiing or like mechanical methods. Abetter polish, however, is achieved electrolytically as, for example, inaccordance with the process described and claimed in the copendingapplication Ser. No. 254.888 of James N. Ostrofsky, now Patent No.2,335,354, Nov. 30, 1943, entitled Polishing stainless iron and steel,in which the stainless steel products are made the anode of an aqueouselectrolyte comprising an aliphatic carboxylic acid such as citric acidand a soluble compound having a sulphate radical such as sulphuric acid;or by anodic treatment in an aqueous solution of concentrated perchloricacid as covered in my copending application Ber. No. 319,057, entitledElectrolytic polishing of stainless iron and steel; or by subiecting theproducts to alternating current treatment in a concentrated acid bathincluding a substantial amount of nitric acid as set forth in thecopending application Ber. No. 460,296 of Alexander L. Feild, entitledElectrolytic polishing of stainless steel. After the stainless steelproducts are polished electrolytically. I wash them in clean water so asto remove traces of the electrolyte employed. The products are dried andthereafter present evenly polished metallic surfaces which are inexcellent condition for coating.

In coating stainless steel articles and products, or other alloy steelproducts, in accordance with my invention, I prepare, in a melting vator the like, equipped with a suitable heating unit, a molten salt bathconsisting of or comprising substantial amounts of sodium dichromateand/or potassium dichromate. The bath advantageously is prepared attemperatures substantially below decomposition temperature of the finalmelt. A bath consisting of fused sodium dichromate and/or potassiumdichromate is preferred.

After melting, the bath is adjusted to treating temperature preferablysubstantially below decomposition temperature and usually in theapproximate range of 320 C. to 500 C. I then immerse a stainless steelproduct, for example. in the bath for a period of time usually rangingfrom about 2 to 30 minutes or more. Temperature of the bath preferablyis maintained substantlaily constant throughout the immersion 2.394399of coating achieved. initial finish of an article or period. The productconveniently is left immersed in the bath during the treatment period oris immersed periodically as by dipping. In any event. it appears that astrong oxidizing action is set up and a coating or film rich in oxidesof iron and chromium accordingly forms on the stainless steel product.

The coated product or article is withdrawn from the salt bath and iswashed free of salt and cooled to room temperature. The product orarticle provided has the many beneficial characteristics of stainlesssteel such as its corrosion resistance, hardness and strength, yet vdurable and beautiful black oxide finish in no manner impairs therustless or corrosionresisting properties of the underlying metal.

The oxide film or coating is dense, hard, tough and corrosion resistantand thus does not wear away readily or wipe off and, moreover, is highlyadherent and flexible and does not crack, chip or spall-ofl even uponbeing subjected to flexing or bending stresses. The coating, whetherdull or lustrous black in appearance depending upon initial finish ofthe steel before treatment, is uniform and of stable quality. My blackoxide coating does not fade in atmospheres of varying humidity or as aresult of rubbing. but maintains its initial beauty for a long period oftime even when exposed to the outdoor elements.

In connection with the practice of my process, a molten salt bathcomprising substantially all sodium dichromate is used to good advantageespecially at temperatures ranging between 320 0., the melting point ofsodium dichromate, and 400 C.. the decomposition point of sodiumdichromate. The bath is particularly useful for providing high qualitycoatings on chromiumnickel steel products such as of the 18-8chromium-nickel grade. but also is capable of giving a fine oxide finishon chromium steel products as well as on other alloy steel products. Ifind that the oxidizing strength 01' the bath increases with temperatureincreased below the decomposition point. Where the bath is held in atemperature range above decomposition temperature, it no longer rapidlyproduces coatings on stainless steel, although good coatings have beenachieved by extending the treatment time. In all, I prefer a processingrange of about 36 C. to approximately 380 0., for within this rangesubstantially no decomposition occurs to affect the oxidizing power ofthe bath and a highly satisfactory oxide coating or film is obtained onthe products in about 2 to 15 minutes time.

In coating stainless steel products, such as those of either chromium orchromium-nickel steel, at bath temperatures higher than those achievedpractically in using sodium dichromate alone, a molten bath containingsubstantial amounts of sodium dichromate.and potassium dichromate isemployed to good advantage. The sodium dichromate of the bath is a moreactive oxidizing agent for stainless steel that is potassium dichromateand, therefore, preferably is included in predominating amount; thepotassium dichromate serving primarily to elevate the decompositionpoint of the bath. A bath thus provided has a decomposition pointranging from 400' C. to 500' C. or more depending upon the amount ofpotassium dichromate used. In treating the products, temperatures ofapproximately 400 C. to 500 C. are permissible without encounteringsubstantial decomposition of the bath. To illustrate, a bath composedsubstantially of 80 parts by weight sodium dichromate and 20 partspotassium dichromate was made up and worked, and it was possible toemploy temperatures within the approximate range of 430 C. to 470 C.without encountering decomposition to any material extent. This samebath later was used at 500 0., being worked each day, and after onemonth was still very efiectlve despite showing considerabledecomposition.

Thus it will be seen that there has been provided in this invention anart and product in which the various objects hereinbefore noted,together with many thoroughly practical advantages, are successfullyachieved. It will be seen that the product is strong, durable andcorrosionresistant, and that it is given a permanent and attractiveblack oxide finish in a direct and thoroughly reliable manner without inany way impairing the rustless or corrosion-resistant characteristics ofthe metal. Moreover. it will be seen that in the practice of my process,stable and highly effective chemicals are employed which are readilyavailable and which are handled with considerable ease.

While my process is described as being particularly useful in providingblack oxide finishes on stainless steel articles and products, it willbe understood that the coatings achieved may be employed as a base forsubsequent coatings or films such as paint, enamel, and the like.

As many possible embodiments may be made of my invention and as manychanges or alterations may be made in the embodiment hereinbefore setforth, it will be understood that all matter described herein is to beinterpreted as illustrative and not in a limiting sense.

I claim:

1. The art of blackening stainless steel articles and products whichincludes, treating the stainless steel articles or products by immersingthe same in a molten salt bath consisting essentially of sodiumdichromate and potassium dichromate, the while holding said bath at atemperature in the range of 400 C. to 500 C.

2. The art of blackening stainless steel articles and products whichincludes, treating the stainless steel articles or products in a moltensalt bath consisting essentially 01' one or more salts of the groupconsisting of sodium dichromate and potassium dichromate, said bathtreatment being continued over a period ranging from about 2 to 30minutes time.

3. The art of blackening stainless steel articles and products whichincludes, treating the stainless steel articles or products in a moltensalt bath consisting of one or more salts oi the group sodium dichromateand potassium dichromate at a temperature of 320 C. to 500 C. for aperiod ranging from about 2 to 30 minutes time.

4. The art of blackening alloy steel articles and products whichincludes, treating the alloy steel articles or products by immersing thesame in a molten salt bath consisting of potassium dichromate insubstantial amount and a predominating amount by weight of sodiumdichromate at a temperature of at least 320' C.

5. The art of blackening metallic articles and products which includes,polishing the metallic articles or products and then treating saidpolished articles or products in a molten salt bath consistingessentally of one or more salts of the group consisting of sodiumdichromate and potassium dichromate.

6. The art of blackening alloy steel articles and products whichincludes, electrolytically polishing the articles or products, and thentreating said polished articles or products in a molten salt bathconsisting essentally of one or more salts or the group consisting ofsodium dichromate and potassium dichromate at a temperature of 320 C. to500 C. for a period ranging from about 2 to 30 minutes time.

'7. The art of blackening stainless steel articles and products whichincludes, treating the stain less steel articles or products byimmersing the same in a molten salt bath comprising substantially allsodium dichromate, the while holding said bath at a temperature in therange of 320 C. to 500 C.

8. The art of blackening stainless steel articles and products whichincludes, treating the stainless steel articles or products in a moltensalt bath comprising substantially all sodium dichromate, the whileholding said bath in the range of 360 C. to 380 C.

9. The art of blackening stainless steel articles and products whichincludes, treating the stainless steel articles or products in a moltensalt bath consisting by weight of approximately parts sodium dichromateand about 20 parts potassium dichromate, the while holding the bath inan approximate range of 430 C. to 470 C.

10. A coating bath composition of the character described, consisting ofa major portion of molten sodium dichromate and a minor portion ofmolten potassium dichromate.

11. A molten coating bath composition of the character described,consisting, by weight, of substantially 80 parts sodium dichromate and20 parts potassium dichromate heated in a temperature range fromapproximately 430 C. to 470 C.

12. In manufactures of the class described, an alloy steel product ofhigh chromium content having an adherent, flexible, black oxide surfacefilm thereon, said oxide consisting substantially of chromium oxideproduced by immersion in a molten salt bath consisting of salt of thegroup consisting of sodium dichromate and potassium dichromate.

13. In manufactures of the class described, a chromium-nickel stainlesssteel product havin an adherent, flexible black oxide surface iilmthereon, said oxide consisting substantially of chromium oxide producedby immersion in a molten salt bath consisting essentially of sodiumdichromate.

14. In manufactures of the class described, a chromium stainless steelproduct having an adherent, flexible black oxide surface film thereon,said oxide consisting substantially of chromium oxide produced byimmersion in a molten salt bath consisting of substantial amounts ofsodium dichromate and potassium dichromate.

IRVINE C. CLINGAN.

